Years of calistentics/gymnastics.
My arch is like that. The first time I did it BigJim told me I had to move further up the bench cause my arch made me end up to far down the bench lol
If your looking at getting flex like this Deep, I would spend 5 mins a day doing back bends. If you can't do them then start by laying on your bed slide to the edge until your bum is just about at the edge then just lean back so that the top of your head is touching the floor. Or if you have one of those huge exercise balls lay on your back with your feet flat on the floor and try and touch the floor (ball in the arch of your back).
Hope this helps